In packet-based, multi-user cellular communication systems, such as multi-user OFDM (Orthogonal Frequency Division Multiplexing) systems, a scheduler-device that makes decisions as to which user is assigned which radio resources and when is typically employed. From time to time, the users report the quality of their respective radio channels to the base station, whereupon the base station makes a scheduling decision. In uplink, the base station measures the channel quality, e.g., from pilot signals transmitted by the users. The scheduler may exploit the fact that the users' channels change independently from each other, i.e., channels of one or more users may be fading, or, also, one or more channels allocated to a specific user may be fading, while others are not. Typically, a user is assigned radio resources when its channel conditions are good. Accordingly, the scheduler improves the performance of the system (in terms of cell throughput) as compared to systems that do not exploit the users' channel quality through a scheduler.
The extent to which the scheduler improves the system performance depends in downlink on the quality of the feedback information. Good, detailed and accurate feedback of the channel quality are necessary. There are situations, however, where such accurate and timely feedback is not possible. A user may, for instance, move at such a high speed that a channel quality measure is outdated and obsolete by the time it reaches the base station. Another example of unreliable feedback measures occurs when a cell-edge user has bad signal-to-noise ratio on the uplink feedback channel and the quality measure is simply detected erroneously at the base station.
Scheduling may also be inefficient if the channel varies considerably in time during a transmission time interval, i.e. in case of high Doppler spread. For certain types of data scheduling may not be desirable, e.g. for data with low latency requirements and low data rates. Feedback data is a typical example of such kind of data. In this case, dedicated channels are more appropriate.
For these situations, the system may provide a frequency-distributed channel in order to provide a high-diversity link-performance. Accordingly, users with reliable channel quality feedback and with data appropriate for scheduling are assigned radio resources when and where their respective channel conditions are known to be good, other users are assigned frequency-distributed channels.
A problem, however, is how to provide both high link-diversity (frequency-distributed) channels and high multiuser-diversity (frequency-localized) channels at the same time in an efficient way.
An attempt to solve this problem is disclosed in IEEE Std 802.16-2004, “Standard for Local and Metropolitan Area Networks”, Part 16: “Air Interface for Fixed Broadband Wireless Access Systems”, 2004, wherein the above problem has been solved through the use of so-called ‘zones’. A zone is a time period during which a certain type of channel is transmitted. Each radio frame contains two zones, one for the transmission of frequency-localized channels followed by one for frequency distributed channels in a pure time-multiplexing fashion. In the header of each radio frame information is conveyed as to when in time one zone changes into the next.
A disadvantage with this solution, however, is that the link-diversity is limited.
Accordingly, there is a need for a system and method with improved link-diversity.